This invention relates to a phase lock circuit comprising a phase frequency comparator having a first input for a reference signal and a second input for a feedback signal, as well as outputs for furnishing an up-signal or a down-signal to a control charge pump circuit, which control charge pump circuit is connected with the outputs of the phase frequency comparator and has an output which is connected via a loop filter with a voltage-controlled oscillator provided with tuning means, the output of the oscillator being connected via a frequency divider with the second input of the phase frequency comparator.
Such a phase lock circuit is known from practice. A similar circuit is also described in EP-A-0 945 986.
A high-quality voltage-controlled oscillator with a low phase noise is tuned with the aid of a varactor. A problem occurring in the known phase lock circuits and in particular with integrated phase lock circuits is that in many applications the supply voltage is too low to realize the tuning voltage range needed for the varactor. In applications for, for instance, pagers, cordless phones, cellular phones, etc., often a 1.5 V battery is used, while for a varactor a tuning voltage range of, for instance, 0.5-3 V may be required.
Heretofore, this problem was solved by the use of a so-called step-up converter, which can generate a supply voltage which is higher than the battery voltage. A drawback of this solution, however, is that it requires a coil of high self-induction, which cannot be formed on a chip. This results in an increase of the number of required components and the manufacturing costs of the circuit, as well as of the space occupied and the chance of parasitic effects as a result of the necessary connections between the circuit section on the chip and the off-chip coil.
Instead of a step-up converter, a charge pump circuit could be used to generate a voltage higher than the battery voltage. An example of such a charge pump is described in IEEE JSSC Vol. 33 No. 4, April 1998. A problem inherent to that solution is that such a charge pump circuit must have a lower output impedance than the, already low, output impedance of the low-pass filter, which loads the output of the phase frequency comparator.
A need therefore exists for a phase lock circuit which on the one hand furnishes a sufficiently great tuning voltage range for the voltage-controlled oscillator but on the other hand does not require any components which cannot be made on a chip in integrated form, and whereby, moreover, problems with regard to internal impedance adjustment are avoided.
The object of the invention is to provide for the need outlined. To that end, according to the invention, a phase lock circuit of the above type is characterized in that for furnishing a tuning voltage for the tuning means a voltage charge pump circuit is provided, which is combined with the control charge pump circuit to form a single charge pump circuit connected with the output of the phase frequency comparator.
In the following, the invention will be further described with reference to the appended drawing.